Rapid pump-down vacuum chambers incorporating cryopumps



Dec. 23, 1969 w. H, HOGAN 3 ,0

RAPID PUMP-DOWN VACUUM CHAMBERS INCORPORATING CRYOPUMPS Filed Oct. 2'7,1966 2 Sheets-Sheet 1 6O Wolrer H. Hogan INVENTOR.

/EWMJ a Arwrney Dec. 23, 196-9 I w, H. HOGAN 3,485,054

RAPID PUMP-DOWN VACUUM CHAMBERS INCORPORATING CRYOPUMPS Filed Oct. 27,1966 2 Sheets-Sheet 2 88 Walter H. Hogan INVENTOR. Fig '6 Z UnitedStates Patent 3,485,054 RAPID PUMP-DOWN VACUUM CHAMBERS INCORPORATINGCRYOPUMPS Walter H. Hogan, Wayland, Mass., assignor to CryogenicTechnology, Inc., Waltharn, Mass., a corporation of Delaware Filed Oct.27, 1966, Ser. No. 590,061 Int. Cl. F25b 19/00 US. Cl. 62-555 19 ClaimsABSTRACT OF. THE DISCLOSURE A rapid pump-down vacuum chamber suitablefor rapidly reducing pressures in a test volume'from atmospheric toabout 10 torr. A cryopump chamber containing a refrigerated thermal massis associated with a work chamber. By effecting fluid communicationbetween the two chambers, the pressure rapidly reduced in the systemthrough the mechanismof cryopumping.

This invention relates to vacuum chambers and more particularly thosewhich are designed to operate in the region between atmospheric and torror lower,

and which employ cryopumping.

Many different systems are available for providing relatively highvacuums for industrial processing and for testing chambers. In some ofthese systems Which require the attainment of a high vacuum, the timeconsumed in pumping down the vacuum chamber is not of great importance.However, there are a number of applications for vacuum chambers where itis desirable, if not necessary, to be able to achieve very rapidpumpdown to the vacuum level desired. For example, in space simulationchambers there is a need to be able to simulate the rapid pressurechange that space vehicles experience as a consequence of rapid ascentin going from atmospheric pressure to pressures in the range of 10-torr. Ideally, this decrease in pressure should be attained in aboutfour minutes accurately to simulate the ascent of a space vehicle. Insome special industrial processes, it is necessary to treat items in anevacuated atmosphere, the steps being to place them in a work chamber,evacuate, treat and then remove. This means that the major amount oftime is spent in evacuating the work chamber. As an example, manyplastic items are metallized on the surface; and this is done by theprocess of vacuum deposition. In such cases, the time consumed inevacuating the working chamber represents a major portion of the timerequired to process the items. If the pumpdown time of the chamber canbe materially reduced, the cost of such operation can also be materiallyreduced.

Mechanical pumps exist which are capable of achieving rapid pumpdownfrom atmospheric pressure to several torr (mm. of Hg) of pressure. Thereare also many pumping means for achieving rapid exhaust from thesub-torr level. These latter include diffusion pumps which operate from10 torr, ion pumps from 10- and sublimation pumps from 10 torr. However,there is no known mechanical device which is capable of achieving rapidpumpdown in the pressure range between several torr and about 10 torr.

A great deal of effort has been devoted in recent years to thedevelopment of cryopumps in which there is provided a refrigeratedsurface on which gas molecules are condensed and sorbed. Recently,cryopumping has been used to pump below 10" or 10 torr and supplementedwith ion or diffusion pumps to remove the noncondensables such as neon,hydrogen and helium. Cryopumps are usually sized for the pumping speedrequired "ice in the ultimate perssure required, and little, if any,consideration is given to attaining a rapid pump-down. This in turnmeans that the refrigerator or the refrigerating system used with thecryopump is sized for the load at the highest starting pressure. Forexample, a suitably sized cryopump for 10,000 liters/ second startingpumpdown at 10 torr will have a radiation load of about 0.05 watt and acondensation load of about 0.05 watt-a total of 0.10 watt. Starting at10' torr, the pump will be the same physical size but the loads will be0.05 watt for radiation and 5.0 watts for condensation, or a total of5.05 watts refrigeration load. This refrigeration load is raised to50.05 watts if the pump must begin to operate at about 10 torr. Inpatent application Ser. No. 521,082, filed in the names of Walter H.Hogan. and Raymond W. Moore, Jr., and assigned to the same assignee asthe present application, the problems noted above which are inherent incryopumps have been materially overcome by providing an automaticallyactuatable, variable-size cryopanel. This in turn effects a reduced loadand pumping speed at higher pressures to match the refrigerating load tothe torr-liter/ second load rather than to the liter/ second load. Thepanels for such a cryopump are typically made as light in weight aspossible to reduce the mass and thus the cool-down load. Cryopanels ofthis type make possible the use of small cryogenic refrigerators insmall cryopumps having pumping speeds in the range of to 100,000liters/second. However, none of the mechanical or cryogenic devices usedto attain high vacuums have been directed to, or solved the problem of,very rapid pump-down.

It is therefore a primary object of this invention to provide improvedvacuum apparatus capable of achieving rapid pump-down. It is anotherobject of this invention to provide vacuum apparatus of the characterdescribed which is particularly suitable for space simulation chambersand some industrial processes. It is another primary object of thisinvention to provide an improved method of evacuating a chamber at avery rapid rate, the method comprising a combination of mechanical andcryogenic pumping steps. Other objects of the invention will in part beobvious and will in part be apparent hereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and theapparatus embodying features of construction, combinations of elementsand arrangement of parts which are adapted to effect such steps, all asexemplified in the following detailed disclosure, and the scope of theinvention will be indicated in the claims.

In contrast to previous cryopump designs, the vacuum system of thisinvention incorporates a cryopump having a relatively large thermal masswhich is matched to the cryopumping load. A separate cryopumping chamberand a working chamber are provided; and they are in fluid communication.They are, however, completely controllably isolated as will be apparentin the following description. In the cryopumping chamber, there isprovided a large thermal mass, cooled by appropriate refrigeratingmeans, which is characterized by having an extensive heat transfersurface area. After the working chamber has been evacuated to a level ofseveral torr (e.g., 1 to 10 mm. Hg) by appropriate mechanical means, thevalve which controls the communication between the two chambers isopened and gas from the working chamber enters the cryopumping chamberand is immediately condensed upon the large surface area of the thermalmass thus rapidly reducing the pressure. The temperature of the thermalmass is raised by several degrees but not to the extent that anyappreciable quantity of the condensed gases boils olf. Refrigeration tothe cryopumping surface is continued and external pumping may also becontinuued for as long as desired. Prior to raising the pressure in thework chamber for withdrawal of test specimens, work pieces or the like,the valve between the work chamber and cryopump chamber is closed. Thethermal mass is cooled again to its lower temperature level inpreparation for re-evacuation of the work chamber, Periodically, thecryopump chamber may be warmed up and pumped out by means of a suitablemechanical pump.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which:

FIG. 1 is a longitudinal cross section through one embodiment of avacuum chamber system of this invention having a refrigerator built intothe system;

FIG. 2 is a cross section along line 22 of FIG. 1;

FIG. 3 is a cross section along line 3-3 of FIG. 1;

FIG. 4 is a fragmentary cross section of the lower part of thecryopumping chamber showing the use of externally supplied refrigerantsin place of a refrigerator;

FIG. 5 is a longitudinal cross section of another embodiment of thevacuum system of this invention; and

FIG. 6 is a diagrammatic sketch of a vacuum system employing multiplework chambers with a single cryopumping chamber.

The embodiment shown in FIG. 1 has the working chamber in line with thecryopump chamber, and a builtin refrigerator. It also illustrates theuse of a solid material such as lead as the thermal rnass. In FIG..1 thevacuum-tight housing 10 having an attaching flange 11 defines a workingchamber 12. It will be appreciated that the working chamber 12 may be ofany size and configuration, the choice being determined by the use ofthe vacuum system. Its representation in FIG. 1 as an essentiallyhemispherical chamber is only exemplary and is to be considered asillustrative only. A fluid exhaust conduit 13 communicates, through abranch fluid conduit 14, with a mechanical vacuum pump 15; and, througha branch fluid conduit 17, with a vacuum pump 18 suitable for pumping inthe sub-torr range. The pump 18 may be a diffusion, ion, or sublimationpump. Valves 16 and 19 are located in branch fluid conduits 14 and 17,respectively. Main vacuum valve 20 controls the flow of fluid betweenthe working chamber 12 and the cryopump 26. This may be a gate valve, orany other suitable valve used in vacuum systems. The valve 20 is locatedwithin a valve housing 21 which has a flange 22 for achieving afluidtight seal with flange 11 of the vacuum-tight housing 10 throughthe use of suitable means such as screws 23. The valve housing 21 isalso equipped with a lower flange 24 for attachment to the cryopump 26,and the opening and closing of valve 20 is achieved through suitableactuating means shown schematically at numeral 25.

It will be appreciated that throughout the description of the apparatusof this invention it is necessary always to form vacuum-tight sealsbetween the various components. Since there are many ways in which thismay be done, e.g., elastomeric O-rings, welding, etc., such seals willnot be shown in detail.

The cryopumping section 26 is seen to comprise an outer cryopump housing28 which defines within it a chamber 29 in which the main cryopump islocated, Within this outer cryopump housing 28 is located the cryopumpchamber housing preferably constructed of a material such as thinstainless steel. It defines the actual cryopump chamber 36, and it isspaced from and affixed to the outer housing 28 and the valve body by asuitable spacing ring 37. A radiation shield 38 surrounds the lowerportion of the cryopump chamber housing 35 and with it defines the lowerportion to the cryopump chamber 36, there being fluid communicationthrough openings 39. The cylindrical wall 35 defining the upper portionof the cryopumping chamber has a bottom plate 40 which serves as aportion of the thermal mass and which supports the remaining portion ofthe thermal mass illustrated here as comprising extended heat-exchangesurfaces in the form of concentric cylindrical configurations 41 (seealso FIG. 2).

The cryopumping surfaces 40 and 41 and the radiation shield 38 arecooled by suitable refrigeration means which in FIG. 1 is shown tocomprise a refrigerator'45 built into the cryopumping section. Asuitable refrigerator for this purpose is described in US. Patent3,218,815 (seein particular, FIG. 6 of that patent). Such arefrigerator, shown as a stepped refrigerator comprising sections 46 and47 in FIG. 1, is capable of delivering refrigeration at two temperaturelevels, corresponding to heat stations 68 and 85 of the refrigerator ofFIG. 6 of US. Patent 3,218,815.'Refrigeration at the lower temperaturelevel (e.g., from 14 to 20 K.) is provided at the end of section 46which is bonded to bottom plate 40 and is iii-direct heat exchangecontact with plate 40 and through it to the heat-exchange surfaces 41.Inasmuch as the refrigerator is not part of the invention, the auxiliaryequipment of the refrigerator (driving means, refrigerating fluidsource, etc.) are diagrammatically represented byfthe numeral 48.

The radiation shield 38, as illustrated inFIG. 1, has a bottom plate 49which serves as the lower wall of the cyropumping chamber and as thethermal connection to the refrigerator. Refrigeration to the radiationshield is delivered at the higher temperature level (e.g., about 70 to80' K.). Using the refrigerator of FIG. '6 of US. Patent 3,218,815 as atypical suitable refrigerator, the radiation shield would be bonded inheat exchange relationship to heat station 68 which draws itsrefrigeration from expansion chamber 50. The remaining portion of theradiation shielding comprises a cylindrical section 50, an upper annularring 51 which passes through cryopump housing wall 35 and makes afluid-tight seal therewith, and an inclined ring 52 which extends withinthe cryopump chamber 36. The radiation shield is closed by means of atop plate 53 supported on the inclined section 52 throughheat-conducting supports 54 (see also FIG. 3).

Inasmuch as it may be necessary periodically to pump out the cryopumpchamber 36, a mechanical vacuum pump 42 may be connected to the cryopumpchamber 36 through a fluid conduit 43, controlled by valve 44.

That volume which is defined between the inside wall of the outerhousing 28 and the outer walls of the inner housing 35 and radiationshielding bottom plate 47 and walls and 51 serves as insulation for thecryopump chamber. As such it may be evacuated, or it may be filled withfinely divided particulate insulation or with a foamed-in-placeinsulation. Many suitable insulating systems are known for this purpose.

It is also within the scope of this invention to provide refrigerationto the cryopumping thermal mass and to the radiation shielding by thecirculation ofcryogenic fluids, such as in heat-exchang relationshipwith bottom plates 40 and 49. This is illustrated in FIG. 4. Coils 55,made of a suitable materials such as stainless steel, are bonded in heatexchange relationship with bottom plate 40 of the thermal mass. Heliumgas at temperature of about 15 K. to 20 K., or liquid helium, is thenintroduced into the coils by means of line 56 and withdrawn through line57. These lines are seen to be insulated with a suitable insulation 58.In a similar manner liquid nitrogen is circulated in coils 60 which arebonded in heat exchange relationship to bottom plate 49 of the radiationshielding. The liquid nitrogen is brought in through linlet conduit 61and withdrawn as gaseous nitrogen through conduit 62, inlet conduit .61,having a suitable insulation 63.

FIG. 5 illustrates another embodiment of the apparatus of this inventionin which the working chamber is distinct from the cryopump chamber andis connected through an appropriate fluid conduit with the valve beinglocated within this fluid conduit. In FIGS. 1 and 5 like elements arereferred to by like numerals. In the apparatus of FIG. 5 the workingchamber 65 has a flange 66 which is designed to provide the necessaryfluid-tight and vacuum-tight seal with an end cover 67 which has a'corresponding flange 68 suitabl for bolting to flange 66 through screws69. The end cover 67 may be so designed as to swing away from theworking chamber 65 or otherwise be removed therefrom in order to permitaccess to the working chamber 70. As in the apparatus of FIG. 1, theworking chamber 70 is adapted to be evacuated by a mechanical pump 15and by a pump 18 suitable for pumping in the sub-torr range.

The working chamber 70 is in fluid communication with the cryopumpchamber through the conduit 73 which contains vacuum valve 20. Valve 20,in turn, is connected to the cryopump 26 which in the embodiment of FIG.5 comprises an outer housing 75 defining an outer chamber 76 whichcontains the inner cryopump chamber wall 78 extending therein. As in FIG.1, a radiation shielding 38 is used and forms part of the housing forthe cryopump chamber 36. The thermal mass of the embodiment of FIG. 5 ishigh-pressure helium (20,000 to 200,000 p.s.i.). This is contained in aplurality of thinwall stainless tubes 80 communicating with a suitablemanifold 81. Refrigeration is supplied to the manifold 81, and hence tothe thermal mass of high-pressure helium, and to the radiation shieldingby a refrigerator as in FIG. 1. Alternatively, the'tubes 80 may containa fluid having a heat capacity which derives from a phase change, e.g.,from a liquid to a gas. Neon may be cited as an example of such a fluid.

FIG. 6 illustrates diagrammatically how more than one working chambermay be connected to and served by a single cryopumping system 26. Thecryopumping system 26 may be of any of the embodiments illustrated anddescribed and the working chambers 85, 86 and 87 may be of any desiredconfiguration. It is to be understood that mechanical vacuum pumps andvacuum pumps capable of pumping in sub torr ranges are to be supplied asdiscussed in detail in th description of FIG. 1. A single main fluidconduit 88 and branch conduits 89, 90 and 91 provide the necessary fluidcommunications which are controlled by main valves 92, 93 and 94,respectively. In the apparatus of FIG. 6 the working chambers 85, 86 and87 may be connected in sequence with the cryopump 26, or they may bserved simultaneously. The use of a multiplicity of separate anddistinct working chambers permits more flexibility in operation andprovides, in general, more efficient and continuous use of the cryopump.

The operation of the vacuum system of this invention may be describedwith reference to the apparatus of FIG. 1. To begin the operation, valve20 is closed, thus closing off the fluid communication between theworking chamber 12 and the cryopump. This then permits access to theworking chamber and the insertion of test specimens or items to beprocessed within the chamber. Subsequent to the effecting of afluid-tight seal between the work chamber and the valve body, valve 16is opened to the mechanical vacuum pump 15 and the work chamber isevacuated to a pressure ranging between a few torr and a fraction of atorr. Valve 16 is then closed, and .main vacuum valve 20 is opened. Atthis point in the process, the thermal mass comprising the bottom plate40 and the extended surfaces 41 has been cooled by refrigeration toabout 15 to 20 K. Only a few seconds are required to condense a largeportion of the gaseous molecules (which have entered the cryopump 36from the work area 12) on the extended surfaces of the thermal mass thusreducing the pressure within the entire system to about torr or evenlower. In this condensation the temperature of the thermal mass israised to about 30 K. Once this pressure level of about 10 torr has beenreached, the heat load on the thermal mass is small and therefrigeration delivered by the refrigerator may be used to cool down thethermal mass from its upper temperature of about 30 K. to

to 20 K. If pressures of less than about 10* torr are required in theworking chamber 12 then valve 19 is opened and pumping is continued bythe pumping means 18 which, as explained above, may be an ion pump, adiffusion pump or a sublimation pump. In this operation it will be seenthat each of the different types of vacuum pump means has been employedover that range in which it operates most efficiently and most rapidly.For example, the mechanical pump 15 is capable of pumping out the workvolume .12 to a few torr in a matter of a few seconds or a few minutes.The cryopump then reduces the pressure of the order of l0 torr veryrapidly and finally the ion pump or diffusion pump can remove thenoncondensables, e.g., hydrogen and neon, from the work area at arelatively rapid rate.

At the end of the test period or processing period, valve 20 is againclosed, air or some other gas is introduced into the work chamber 12 andit is returned to atmospheric pressure. The cryopump, on the other hand,remains at reduced pressures and the cryopumping thermal mass isreturned to its lowest temperature if this has not already beenaccomplished.

Two typical applications of this vacuum system may be given toillustrate its operation and use.

Assume in the first example that a work chamber having a volume of44,000 cubic feet is to be used to simulate the very rapid ascent of aspace vehicle and that it is required to pump this chamber down to about10" torr in four minutes. Rough evacuation to a fraction of a torr,e.g., 0.38 torr, or /2000 atmosphere, can be achieved by mechanicalpumps in approximately two minutes, leaving two minutes to pump down to10* torr. This, in effect, requires at this stage the attainment of apumping speed of about 100,000 liters per second. N

If a conventional, or prior art, cryopump were to be used to providecryopumping at this required rate, it would be necessary to use acryogenic refrigerator having a capacity of several thousand watts.However, once steady cryopumping at 10* torr, or lower, is attained, therefrigeration requirement drops drastically to only a few watts whichmeans that the remaining capacity of the refrigerator is in effectwasted.

In contrast to the refrigerator capacity demanded by prior art cryopumpsto attain the rapid pump-down, a vacuum system constructed according tothis invention would require a refrigerator having a capacity of about120 watts if as little as one hour is allowed between successivepump-downs. This is due to the fact that the refrigeration required overthe two-minute pump-down from a fraction of a torr to 10 torr isfurnished by the thermal mass, and not by the refrigerator. To condensethe air in the work chamber of this example from, say, 0.38 torr (Vatmosphere) requires refrigeration of 370 B.t.u. or 108 watt hours. Theheat capacity of lead (the thermal mass) between15 K. and 30 K. is about0.37 B.t.u. per pound, so that about 1,000 pounds of lead initially at15 K. (a little over a cubic: foot), presenting a surface area between40 and square feet, would warm up to 30 K. while condensing a mass ofair equivalent to the 22 standard cubic feet which must be removed toattain 10 torr in this chamber. The initial heat flux would be about20,000 watts and the average over the two-minute pump-down would beabout 3,000 watts. The heat load at the end of this period, when thepressure is down to 10- torr, is less than 1 watt due to condensationand radiation; and it may be about 10 watts due to conduction heat leaksdown the support structure for the thermal mass. Since the initial heatload in condensing out the condensable constituents of the gas at 0.38torr is taken by the heat capacity of the thermal mass, the refrigeratorused need only be sized for the average duty placed upon it to cool downthe thermal mass within a specified time period and to maintain steadystate cryopumping, i.e., that which is required by virtue of heat leaksinto and radiation heat transfer within the cryopump.

In the vacuum system of this example, assume further that it is requiredto repeat the pump-down of the working chamber from atmospheric pressureto 10- torr every hour. Thermodynamic calculations show that 108 Wattsof refrigeration would be required to cool the thousand pounds of leadfrom 30 K. to 15 K. in the hour interval and that an additional one to10 watts are needed to meet steady-state load requirements over thathour. 'Hence, the refrigerator must have a capacity of about 120 watts.If it were necessary to repeat this pumpdown only once every ten hours,then similar calculations show that a refrigerator having a capacity offrom about 10 to 20 watts is adequate.

For many uses, it may not be necessary to use mechanical pumping at all.As a second example of the operation of the vacuum system of thisinvention, assume that a small working chamber is to be rapidlyevacuated. Typically, such a chamber is a bell jar which may have avolume of about two cubic feet. Using the figure of 16.8 B.t.u./standard cubic foot as the energy required to condense the condensableconstituents in air to attain a vauum of about l torr, it follows thatthe total refrigeration load is 33.6 B.t.u. This could be adequatelyprovided by about one hundred pounds of lead having a surface area ofabout one square foot. The pump-down, once the main vacuum valve isopened, is almost instantaneous and the noncondensable residual gasescan be rapidly removed by suitable pumping means well known in the art.

Once the choice of the thermal mass material (e.g., lead, copper,high-pressure helium or a fluid capable of experiencing a phase changein the cryogenic temperature range involved) has been made, the weightof the thermal mass may be readily calculated from its known physicalproperties to meet any desired performance characteristics. In general,the thermal mass may be defined as that quantity which is required toprovide essentially all of the refrigeration required for the transientpump-down condensation to about torr whether it begins at atmosphericpressure or at a pressure in the range of a fraction of a torr to a fewtorr. For all practical purposes, this amount of thermal mass will be atleast 10 times that which would be required if the refrigeration weresupplied directly from the refrigerating means rather than stored in thethermal mass.

Finally, the operation of the vacuum system of this invention may bedescribed in terms of a comparison with the prior art approach as it isdescribed in the literature. (See for example The Journal of VacuumScience and Technology, 3, No. 5: 252257 (September-October 1966).) Fromthis reference, temperature levels, refrigeration capacity andcryo-array geometry requirements can be determined. For example, a100,000 liter per second pump to operate at a steady state pressure of10- torr would require about 40 square feet of condensing surface.Panels making up square feet arranged to condense on both sides couldeffect this speed. In order to minimize temperature differences from onepart to another of this panel, and to insure the panel is adequatelyrigid, a reasonable choice for panel material would be to ,4 inch thickhigh purity copper. The weight of these panels would be only 55 to 110pounds, with a heat capacity between 15 K. and K. of 4.4 B.t.u. to 8.8B.t.u. If this pump were used in the transient mode as described in thefirst example of the operation of the vacuum system of this invention,it would, if the cryopanels were initially cooled to 15 K., be able topump down the system to 10 torr only if cryopumping began at a pressureno higher than 5 10- torr to 10* torr, depending on the mass used,rather than from 0.38 torr as shown in the example. This, of course,does not make it possible to achieve the rapid pump-down through thediflicult region between about 1 torr and 10 torr which is achievedthrough the use of a thermal mass.

It will be seen from the above description and detailed discussion thatthere is provided method and apparatus for rapidly attaining vacuums inthe range of about 10" torr. Moreover, with additional vacuum pumpingmeans the pressures may be lowered to 10 The apparatus is flexible inits operation, relative simple to construct and in some modificationscan be used continuously.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efliciently attained; andsince certain changes may be made in carrying out the above method andin the constructions set forth without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

I claim:

1. A vacuum system, comprising in combination:

(a) a work chamber;

(b) a cryopump chamber containing thermal mass means in the form ofextended surface area, said thermal mass means being present in at leastthat quantity which is capable of providing essentially all of therefrigeration required for the transient pump-down from more than about10 torr, through condensation of gases, to about l0 torr;

(c) refrigerating means associated with said thermal mass means andadapted to cool it to a temperature below that at which the majorconstituents of said gases condense and solidify;

(d) fluid conduit means connecting said Work chamber and said cryopumpchamber; and

(e) valve means located in said fluid conduit means and adapted tocontrol the flow of fluid between said Work chamber and said cryopumpchamber.

2. A vacuum system according to claim 1 wherein said thermal mass meansis at least ten times that required in normal cryopumping whereinessentially all of the refrigeration required for said condensation issupplied directly from said refrigerating means.

3. A vacuum system according to claim 1 wherein said thermal mass meansis formed of a metal having a high heat capacity at cryogenictempera-tures.

4. A vacuum system according to claim 3 wherein said metal is lead.

5. A vacuum system according to claim 1 wherein said thermal mass meansis a fluid capable of undergoing a phase change within the temperaturechange range experienced by said thermal mass means.

6. A vacuum system according to claim 1 wherein said thermal mass meansis high-pressure helium contained in thin walled tubing.

7. A vacuum system according to claim 1 wherein said refrigerating meansis a cryogenic refrigerator forming an integral part of said cryopump.

8. A vacuum system according to claim 1 further characterized by havingmechanical vacuum pumping means in controllable fluid communication withsaid work chamher.

9. A vacuum system according to claim -8 including vacuum pumping meanscapable of pumping in the subtorr region in controllable fluidcommunication with said work chamber.

10. A vacuum system according to claim 1 further characterized by havingmechanical vacuum pumping means in controllable fluid communication withsaid cryopump chamber.

11. A vacuum system, comprising in combination:

(a) a work chamber;

(b) a cryopump chamber containing thermal mass means in the form ofextended surface area, said thermal mass means being present in at leastthat quantity which is capable of providing essentially all of therefrigeration required for the transient pumpdown from more than about10- torr, through condensation of gases, to about 10- torr;

(c) radiation shielding means surrounding at least that portion of saidcryopump chamber containing said thermal mass means;

((1) outer housing means surrounding said cryopump chamber and saidradiation shielding means and adapted to provide thermal insulationtherefor;

(e) refrigeration means adapted to cool said thermal mass means to atemperature below that at which the major constituents of said gasescondense and solidify and to cool said radiation shielding means;

(f) fluid conduit means connecting said work chamber and said cryopumpchamber; and

(g) valve means located in said fluid conduit means and adapted tocontrol the flow of fluid between said work chamber and said cryopumpchamber.

12. A vacuum system according to claim 11 wherein said refrigeratingmeans is a cryogenic refrigerator capable of delivering refrigeration attwo temperature levels, the lower level being used to refrigerate saidthermal mass means and the upper level to refrigerate said radiationshielding means.

13. A vacuum system according to claim 11 including mechanical vacuumpumping means in controllable fluid communication with said workchamber.

14. A vacuum system according to claim 12 including vacuum pumping meanscapable of pumping in the subtorr region in controllable fluidcommunication with said work chamber.

15. A vacuum system according to claim 11 including mechanical vacuumpumping means in controllable fluid communication with said cryopumpchamber.

16. A vacuum system, comprising in combination:

(a) a plurality of work chambers;

(b) a cryopump chamber containing thermal mass means in the form ofextended surface area, said thermal mass means being present in at leastthat quantity which is capable of providing essentially all of therefrigeration required for the transient pumpdown from more than abouttorr, through condensation of gases, to about 10- torr;

(-c) refrigerating means associated with said thermal mass means andadapted to cool it to a temperature below that at which the majorconstituents of said gases condense and solidify;

(d) fluid conduit means connecting each of said work chambers with saidcryopump chamber; and

(e) valve means located in said fluid conduit means and adapted tocontrol the flow of fluid between each of said work chambers and saidcryopumping chamber.

17. A method of rapidly reducing the pressure of a gas mixture within achamber from atmospheric down to about 10- torr, characterized by thestep of causing the condensable constituents in said gas mixturesuddenly to contact previously cooled thermal mass means whereby saidcondensable constituents are solidified on the surface of said thermalmass means, said thermal mass means being cooled to a temperaturesufliciently low so that its temperature subsequent to thesolidification of said condensable constituents remains below that atwhich said constituents are solidified.

18. A method in accordance with claim 17 wherein said thermal mass meansis cooled to between 15 and 20 K. prior to said sudden contacting andrises to a temperature no higher than about 30 K. during thesolidification of said condensable constituents thereon.

19. A method in accordance with claim 17 wherein said step of suddenlycontacting is preceded by mechanically pumping to reduce the pressure insaid chamber down to a range between a few torr and a fraction of atorr.

References Cited UNITED STATES PATENTS 3,144,200 8/ 1964 Taylor 62-5553,168,819 2/1965 Santeler 62-555 3,252,652 5/ 1966 Trendelenburg 62-555MEYER PERLIN, Primary Examiner US. Cl. X.R. 62-268

